MFJ-949E Capacitance and Inductance Settings

Like many amateur radio operators, I use an "Antenna Tuner" to create a good match between my HF transceiver and the feeder and antenna. Really, the best place for a matching device is at the terminals of the antenna, because this ensures a low VSWR on the feeder cable, and not just the output of the transmitter.
However, most of us find it much more convenient to have the tuner alongside the radio, and tolerate the additional loss which having a high VSWR on the feeder causes.

The tuner that I use is a manual one, an MFJ-949E. I have had it for many years and it seems reliable and well made. It has a number of useful features such as a built-in VSWR meter, and a dummy load, but the part which creates the good match between the radio and the antenna system consists of just three components. MFJ provide a circuit diagram in the back of the manual.

The circuit is a "T" filter, with two variable capacitors and a switched inductor. The design has varied a little bit over the years, and I have seen an older version which had a kind of sliding wiper on the inductor to vary its value.

The MFJ circuit diagram shows C1 and C2 are 208pf, but no values are given for the inductor. I thought it would be nice to measure the values of the capacitors and inductors at each position, because this would allow for some analysis of the antenna impedance. The impedance of the matching device should be the "complex conjugate" of the antenna impedance. Which basically means the reactance will be equal and opposite. But the situation is made more complex as the length of the feed line will transform the values. A 1/4 wave line will make the values opposite, and after a half-wave they will be the same again, repeating along the length of the line. If you want to know more about this look up the "Smith Chart" which allows this transforming effect to be calculated easily.

Transmitter / Antenna Control	Capacitance Value (pF)	Inductor Control Position	Inductance Value (μH)
0	221.6	A	31.91
1	202.5	B	15.52
2	180.7	C	9.888
3	163.6	D	6.843
4	141.2	E	4.690
5	119.9	F	2.835
6	101.7	G	2.015
7	80.0	H	1.331
8	60.5	I	0.744
9	39.2	K	0.329
10	26.3	L	0.120

The Transmitter and Antenna capacitors C1 and C2 appear to be identical. The controls are marked 0 to 10, but lining up the dot on the knob with the number is rather approximate.

The table shows that the capacitors have a linear law, and that there is about 20pF between each step from 220 down to 20pF. The inductor is a bit more "logarithmic" in its steps, but the switch makes the setting precise and repeatable.
I find it counter-intuitive that the capacitor numbers and inductor letters get larger as the values get smaller.

I measured these values on a DER EE,  LCR meter DE-5000, with the measurement frequency set to 100kHz.

73
Hugh M0WYE

Throat Microphone for bicycle mobile

I wanted to try using a throat microphone while operating the radio mobile on a bicycle. The big problem is wind noise on any conventional mic, but a throat mic is a contact microphone which picks up the vibrations directly through the neck.
Apparently they are often used by security guards and the like for discreet radio communications because they can be hidden under a collar.
I bought one of these.
https://www.amazon.co.uk/gp/product/B075D6PFPD

The two-pin jack plug connects to my TYT MD380 DMR radio, so it was quite easy to try out. The Ashford repeater GB7AS features an echo facility where you can talk for a few seconds, and then the repeater plays back a recording of your transmission. I found that the audio was quite muffled, but was best when I moved the microphone as high up the neck as possible.

But I also noticed another problem. On high power, there was a "puttering" noise on the transmission, which seems to be the radio frequency signal getting into the microphone and causing interference. The noise went away on low power, but was very noticeable on high power, where-ever I positioned the radio antenna.

Undaunted, I took the equipment for a 12 mile cycle ride up onto the North Downs. I was unable to raise anyone on the Charing repeater GB3CK (using analogue transmission), but I did manage to get a signal report from Peter GW6YMS on Anglesea in Wales on DMR, via GB7AS.
"Unintelligible"  was his verdict, and "there's terrible motor-boating on your transmission" - which didn't go away even when I switched to low power. I gave up and talked to him using the microphone in the radio.

The earpiece part of the device does work quite well, although it tends to fall out of my ear after a mile or two. It is quite clear to listen to, private and comfortable to use - and I can still hear traffic coming up behind quite well too.

Curiosity got the better of me and I opened up the microphone and the little "junction box" in the cable, to see what was inside.

 You can see that the "microphone" is just a very small "piezo bender". This seems to be connected directly to the microphone input of the radio. There is a soft pad of foam behind the bender, and a small piece of blank fibreglass PCB. The wiring looks pretty horrible, with joints insulated by little dobs of hot-melt glue.

The junction box is equally ugly inside with more blobby joints and hot-melt glue.

It was very inexpensive.

I might try to make a boom microphone like I use in the car, with an electret capsule in a wind shield of some kind. I could keep the earphone part of this and try and improve the RF shielding.

73 Hugh M0WYE

Sealed Lead-acid Battery Charge Circuit for Jump Starter

On the left is a "Jump Starter" which contains a sealed lead-acid battery and originally had two big cables with crocodile clips to help start car engines when the car battery is flat. I never used it for that, I took off the big leads and added a fused cable to power my FT847 radio for portable operation. Not only does the case make the battery safe and easy to carry, but it also has a built in volt meter and an emergency lamp. On the right hand side there is a "cigarette lighter" type accessory socket to power other items or smaller radios.

The Jump Starter came with a mains adaptor to charge the battery via a power socket underneath the meter.

This is the weak-point of the system, because the adaptor is unregulated and when the battery is fully charged, the voltage reaches over 20 Volts. This has almost certainly caused the premature failure of the original battery, because lead-acid batteries should not be charged to a voltage higher than 14.4 Volts.
If the Voltage is regulated to 13.8V then the battery can be "float-charged" continuously.

There is plenty of room inside the box, and I have wanted to install a Voltage regulator for a long time. But I discovered that I could buy (very cheap!) a little regulator board from ebay.
https://www.ebay.co.uk/itm/LM317-Adjustable-Converter-Voltage-Regulator-Module-Variable-Power-Supply-DC-DC/263896453817

The module came with a piddly little heatsink, but, as the regulator is close to the edge of the board, it is easy to add extra aluminium.

There are just four connections: input +/- (on the left) and output +/- (on the right). The voltage can be adjusted with the multi-turn preset at the bottom of the picture. It is wise to twiddle the adjustment screw to get 13.8 Volts before connecting the battery.

While adding extra heatsinking, I also added a smear of heatsink compound - as there was none on the device.

Now purists will complain that there should really be a current limit incorporated into the charge circuit, because a very discharged battery will draw a high current when charged. And this is true, however the existing circuit includes a one-ohm, high power resistor in series, and also the mains adaptor is rated at 500mA, and simply can't supply too much current for a lead-acid battery of this size. Whether the mains adaptor is really adequate is another matter.

Here is the circuit of the Jump Starter. The fused cable to the radio is connected straight across the battery terminals.

Hopefully the replacement battery will last longer now, although it is a smaller capacity than the original, which was claimed to be 12Ah. I say "claimed to be" as it weighed less than the 7Ah battery that I replaced it with and therefore probably had less lead in it!

Now the meter reads 13.8 Volts when the battery is charging.
This is good.

There is a data sheet for the LM317 regulator IC here:
http://www.ti.com/lit/ds/slvs044y/slvs044y.pdf

The internals are a spaghetti of wires, but it is all well insulated :-).

The LM317 with a piece of scrap aluminium as a heatsink, board is mounted top right.

If you want to be scientific, then the "waste heat" that the regulator has to get rid of is the difference between the input and output voltage multiplied by the current flowing through the regulator. Commercial heatsinks are specified with "degrees per watt" which tells you the amount that the temperature of the heatsink will rise when it is dissipating 1 Watt and can be multiplied by the wasted power to work out whether you can fry eggs on it.

Or you can leave it on for a bit and feel (carefully) whether it has got warm or not. In my case not.
73
Hugh M0WYE

PAM8403 Amplifier

I have some old audio-visual speakers, that were once used on a stand at exhibitions. They are quite battered, but the little 3" drive units seem ok. I thought I might make a pair of book-shelf speakers using them - sort of "PC speakers" that are a bit better quality than my old Sony ones.
I needed a little amp to drive them and spotted various amplifier modules on Ebay and Amazon. The speakers are rated at 3W so this PAM8403 module, at 3W per channel, seemed very suitable. PAM8403 Module from Ebay.

The module is tiny. It runs off 5V and has a stereo volume control with an on/off switch. There are input, output and power connections and that's it.
I hooked it up to a bench power supply, and a pair of the A-V speakers. I connected the headphone jack of my tablet to the input and was straight away listening to music at good volume level.
With everything turned up max, the music was very loud, but not distorting, so I am not sure that I had enough drive level to get the the full 3W. There was about 200mV going in when I looked on the scope.

Of course when I moved the scope to the output all I saw was a big blur! This is a "Class D" amplifier, that works using Pulse Width Modulation.

This is what I see on the speaker output with no signal going in. Here the timebase is set to 1uS per division, and the vertical scale is 2V/division. So this waveform is at 240kHz and is 8V peak to peak.

I guess this is going to radiate like crazy, and if I use this amplifier it may be best to use one module in each speaker and keep the speaker leads really short. Lots of filtering on the power-supply too.

The quiescent current was about 20mA and even when the volume was at maximum it was only drawing about 60mA. So it is incredibly efficient - 90% efficiency if the data is to be believed.

I found the datasheet for the chip here: https://www.diodes.com/assets/Datasheets/PAM8403.pdf

Anyway the amp module looks great for testing out the speakers. I can decide whether to carry on using it later.
73
Hugh M0WYE

Diamond X-30N Capacitor Problem

Many radio hams have one of these white stick antennas. Mine is a Diamond X-30N, which is the smallest of the bunch, being 1/2 wave on 2 meters and 2x5/8 on 70cms. It is perched high on the roof and has given good performance.
I chose the Diamond X-30, because ham radio friend had an X-50, and that had given him trouble free service for many decades.
But mine began to give trouble. The VSWR became very high (over 3:1) , but only on the 2m band, 70cms seemed to be ok. Now it took me quite a while to convince myself that the antenna was at fault, because there was a faulty N-connector, but after several trips to the roof, I eventually decided that the antenna was not right. I think what swung it was realising that the 2m signal from the X-30 was nine "S" points down on a 1/4 wave vertical I had set up to compare it with. There was definitely a problem.

I set it up on a pole in the garden and found the VSWR to be rather variable, but generally around 5:1. Whereas the 70cms VSWR was about 1.2 to 1.

At the base of the antenna is a hex-socket grub screw, and if you loosen it, you can slide the internal gubbins out of the fibreglass "raydome".

Inside, everything was clean and shiny, with no sign of damage or water ingress. There is a coil at the base, with a small capacitor tapped on to it..

Now I was surprised how small the capacitor was, given that this aerial is supposed to handle 150 Watts. The capacitor is in series with the centre pin of the connector, so all the power has to go through the little cap. My suspicions were further aroused when I started reading on the internet that other people have had problems with this capacitor failing. The capacitor is marked 500V (it's Voltage rating) and 5C. 5 is the value in pF (i.e. 5pF) and C is the tolerance. The black tip to the capacitor means that it is an "NPO" type, which means it has a very low temperature coefficient.
Now 500V sounds like a lot, but you have to remember that is a d.c. value, and there is a significant derating to apply as you go up in frequency.

Note that some models of this antenna have two capacitors.

I took the capacitor off an measured it, and it actually measured 5pF, but I didn't trust it - ceramic dielectrics can be very peculiar things. I didn't like it - but what could I replace it with - there is no room for a transmitter type capacitor. I wondered about making a capacitor out of double-sided PCB material or coaxial cable ...
After looking around on the internet, I came across PA0FRI's excellent Website . It is hard to give a URL for an individual page, so you will have to follow his links to "antennas", and then to his modification of a diamond X-510, where he replaces the little ceramic capacitors with coax capacitors made from PTFE coax.
I remembered that I had some of that, so I decided to make one. The coax I used was RG178.

You can get the dimensions off the photo at left, but it is best to strip one end then trim it down to length using a capacitance meter. Remember to subtract the stray capacitance of the leads from the value that you read, and use very short leads on the meter, 5pF is almost nothing!

There is a complication in that I have removed the screen at the other end of the cable for several millimeters. This reduces the chance of a high-voltage breakdown at the end. I also put a blob of hot-melt glue on the end and covered it all over with heatshrink tube, so there was no chance of shorts to the coil in the antenna. But it makes trimming to length harder, and in fact I made a practice one first, to work out the length, then cut a "proper" one.

The photo shows the finished capacitor., attached to the coil. It folds around the insulator below the coil, and I found there was plenty of room for this in the tube.

Having put it all back together again, I took it outside and tested it in the garden. The VSWR was about 1.5 to 1, so not too bad. 70cms was also still good.

So I put the aerial back up on the roof. Checking the VSWR here gives a much lower figure of about 1.1 to 1 on 2m and about 1.2 to 1 on 70cms. So that is very promising.

I need to do some on-air testing, but in theory the PTFE dielectric should be very robust and stable. I will add a follow-up post when I have tested it more thoroughly.

73 Hugh M0WYE

Hmmm... maybe spoke too soon. The antenna worked well for a while, then went high VSWR again. Next morning it was low for about half an hour, then went high. Clearly an intermittent problem.

Having another look at the lower coil, I realised that the wire was not enamel coated, so uninsulated. Where the turns had been squeezed up, they were just touching. So I opened them out a bit. I also had a look under the blocks of foam  at either end of the middle coil. The middle coil is joined to the brass elements with a crimped brass sleeve. It looked like a good connection, but just to

be sure, I ran some solder over it.

The antenna is back up on the roof. Seems to be working well at the moment. Hopefully it will last this time. It has about a half an "S" point improvement over a 1/4 wave vertical in the same location. I think +3dB is about what you would expect for a 1/2 wave radiator.

73
Hugh M0WYE

Defiant Radio Set

CWS Defiant A55 Germanium Transistor Radio Set

My mother and father purchased a radio set, about the time they were married (around 1960).

I remember it from my childhood as it was always on in the mornings.

The radio is a Defiant brand set made by the Cooperative Wholesale Society. There is quite a lot about this type of radio on the internet, so I won't go into the history here. Mine is an A55 model. 

Link to the "Radio Museum" gives more information.

It belongs to an era when radios had germanium transistors. With hind-sight we know now that was quite short period, only about a decade, from the end of the fifties to the mid sixties. Valved radio sets were regarded as old fashioned, and the silicon transistor had not yet become commonplace.

I have had the radio up in the loft for many years, and last time I tried it, the audio was quite distorted.

Small modern capacitors wired in parallel with the old.

I thought it would be a good idea to replace the electrolytic capacitors. They are the yellow components in the picture at left. They have a red end and a black end to indicate their polarity. 

These components deteriorate with age, because they have a liquid electolyte, The seals can fail and the capacitors dry out. This type of capacitor is often used between sections of the circuit, where they allow the signals to pass through (these are a.c. currents at audio frequencies) whilst preventing direct current from flowing, thus keeping each transistor correctly biased. 

Modern components are very much smaller than the 1960 types, so I decided to leave the old capacitors in place on the board, cutting one wire so they were no longer in circuit. Then I soldered replacement parts on the underside of the board, thus keeping the appearance of the PCB the same, but hopefully restoring full circuit function. 

After replacing all six capacitors, I switched on the set, which initially worked well - but next time I turned it on it didn't. Sometimes it worked, sometimes the audio was weak and distorted. The Defiant has an unusual power-supply arrangement. It was powered from a PP11 type battery, which is fact two 4.5V batteries in the same box. This provided a split rail supply. The circuit runs from 9 Volts, but the speaker is connected to the mid-point of the batteries. This was done to avoid the need for a large electrolytic capacitor in series with the loudspeaker, and ensured a good bass response on the audio. But it also means that the On/Off switch has to be a double-pole type, in order to switch both positive and negative rails. The on/off switch on my Defiant was intermittent on one, or both sides.

On/Off switch seen here in the "off" position, 

 On/Off switch seen here in the "off" position,

On/Off switch seen here in the "on" position, 

And here in the "on" position. 

Rather than replace the On/Off volume control with a modern part, I took the switch off the back of the potentiometer. This was quite easy to do and just required bending some metal tabs. The switch had some old dried grease in it. I used some spray switch cleaner to soften and remove a lot of the grease, but hopefully leave the internals clean and lubricated. The switch works really well now.

I am happy to report that the set now sounds fantastic, on both long and medium wavebands. BBC radio 4 is still broadcasting on longwave in the UK. 198kHz.

The other things I have done are cleaning – the case was very dirty.  I also glued a bit of scrim back over the hole in the back of the cabinet.

The set has 6 transistors, OC45’s in the RF section and an OC75 as an audio driver, with two OC74’s in the output stage. There were also 6 electrolytic capacitors, ranging from 8uF to 250uF.

I have taken the spun brass disks out of the control knobs and polished out the tarnish – then gave them a coat of varnish to prevent them discolouring again.

the disks were quite easy to remove by poking the end of a cocktail stick through the little hole in the plastic moulding. The glue was hard and dry, and I replaced it with a little dab of hot-melt glue.

Flaky paint on the dial

Unfortunately the paint on the rear of the dial is flaking off, which is a great shame and difficult to fix. I don’t think there’s much that can be done there.

Since PP11 batteries are no longer available I am using a 6-pack of AA cells, I connected a wire to the mid point of the battery pack to provide the split-rail supply that I mentioned earlier.

The set has a particularly rich and full sound. Here are a few more photos.

73

Hugh M0WYE

Sound of a Reed Warbler Singing

Recorded the sound of this reed warbler singing in the ditch by the road one day, thought I would share it on here - the bird itself is completely hidden from view, very secretive!
Video of Reed Warbler singing
73
Hugh M0WYE